Apparatus for detecting the oscillation amplitude of an oscillating object

ABSTRACT

Apparatus for detecting the oscillation amplitude of an oscillating object ( 3 ) includes an optical radiation source ( 9 ) and a detector ( 10 ) including first and second optical radiation sensing areas ( 24 A,  24 B) adjacent each other. The detector ( 10 ) and the optical radiation source ( 9 ) are adapted to be located opposite each other with the oscillating object ( 3 ) located between the source ( 9 ) and the detector ( 10 ) so that the object ( 3 ) blocks a portion of the sensing areas ( 24 A,  24 B) from receiving optical radiation from the source ( 9 ). A processor ( 18 ) coupled to the detector ( 10 ) receives first and second output signals representing the magnitude of optical radiation sensed by the first and second optical radiation sensing areas ( 24 A,  24 B), respectively. The processor ( 18 ) processes the first and second output signals to obtain an indication of the amplitude of oscillation of the object ( 3 ).

The invention relates to apparatus for detecting the oscillationamplitude of an oscillating object and in particular, the oscillationamplitude of the capillary tip of an ultrasonic transducer forultrasonic welding.

During the semiconductor packaging process a semiconductor die (or chip)is bonded to a metal leadframe. This is commonly known as dieattachment. Conductive wire is then bonded between electrical contactpads on the die and electrical contacts on the leadframe by a piece ofequipment commonly known as a wire bonder. The wire bonder bonds theconductive wire to the die and the leadframe by an ultrasonic weldingprocess, which uses an ultrasonic wave transducer. The ultrasonic wavetransducer has a capillary working tip mounted on it and the conductivewire passes through a through bore in the capillary to the capillarytip. It is the tip, which applies the ultrasonic vibration from thetransducer to the conductive wire to form the bond. The transducergenerates longitudinal vibration of the capillary tip, which bonds thewire onto the die pad or the leadframe.

The oscillation amplitude of the capillary tip has been identified asone of the critical parameters necessary to achieve consistent bondingresults. Due to the very small size of the capillary tip and the complexvibration pattern, it is difficult to accurately measure the vibrationamplitude of the capillary tip in both free (unloaded) vibration modeand loaded vibration mode. A further complication is that differentcapillaries used in different transducers have different vibrationpatterns. A large number of attempts have been made in recent years todevelop systems to measure the oscillation amplitude accurately.

However, these systems either can not perform real-time measurement orinvolve a complex series of operations in a controlled environment. Withsome of the systems it is even necessary to switch off the wire bonderduring the measurement process.

For example, U.S. Pat. No. 5,199,630 measures the transducer's vibrationamplitude by using an opto-electronic receiver and a correspondingelectronic controller. To perform the measurement, the apparatus must bere-calibrated every time and thus cannot perform real-time measurement.To measure the transducer's vibration, the apparatus must be fixed tothe bonding area of the wire bonder. The apparatus needs to be removedfrom the bonder after measurement for normal operation of the wirebonder. Hence, the apparatus can not be used to measure the oscillationamplitude during an actual wire bonding operation. Therefore, thisapparatus is not practical to conduct frequent amplitude measurements.

This apparatus is also sensitive to the ambient temperature during themeasurement process as it measures only the optical power variation dueto the vibration of the transducer. Therefore, this apparatus is notconvenient to use in an industrial environment where the temperatures inthe vicinity of the capillary tip can be high due to the bondingoperation.

Furthermore, the apparatus disclosed in U.S. Pat. No. 5,199,630 measuresthe oscillation amplitude of the ultrasonic transducer that holds thecapillary tip, not the vibration amplitude of the actual capillary tip.When one capillary is replaced with a new capillary, for example due towear of the capillary or a different capillary is needed to bond a newdevice, the actual vibration of the capillary may be different.

Therefore, the measurement of the oscillation amplitude of thetransducer cannot be used to precisely monitor the quality of the bond,as the oscillation amplitude measured does not accurately reflect theoscillation amplitude of the capillary tip.

In accordance with a first aspect of the present invention, apparatusfor detecting the oscillation amplitude of an oscillating objectcomprises an optical radiation source; a detector comprising first andsecond optical radiation sensing areas adjacent each other, the detectorand the optical radiation source adapted to be located opposite eachother with the oscillating object located between the source and thedetector so that the object blocks a portion of the sensing areas fromreceiving optical radiation from the source; and a processor coupled tothe detector to receive first and second output signals representing themagnitude of optical radiation sensed by the first and second opticalradiation sensing areas, respectively; the processor processing thefirst and second output signals to obtain an indication of the amplitudeof oscillation of the object.

In accordance with a second aspect of the present invention, a method ofdetecting the oscillation amplitude of an oscillating object comprisespositioning an optical radiation source and an optical radiationdetector on opposite sides of the object, the detector comprising firstand second optical radiation sensing areas; illuminating the object withoptical radiation from the source and processing first and second outputsignals from the first and the second optical radiation sensing areas todetermine the oscillation amplitude of the object.

The term “optical radiation” as used herein covers electromagneticradiation in the visible, ultraviolet and infrared regions of theelectromagnetic spectrum.

An advantage of the invention is that it permits the amplitude ofoscillation of a capillary tip of an ultrasonic bonder to be measuredwithout influencing the vibration of the capillary tip. This enablesreal-time measurement of the vibration amplitude of the capillary tip ofa wire bonder and enables the transducer to be calibrated to produceconsistent vibration amplitude of the capillary tip and to therebyimprove the bond quality.

Preferably, the oscillating, object is a tip of an ultrasonic transducerin an ultrasonic welding machine. Typically, where the ultrasonicwelding machine is a wire bonder, the tip is a capillary tip.

Typically, the processor may generate an output oscillation signal,which can be applied to the oscillating object to modify the oscillationamplitude of the object in response to the oscillation amplitudedetected by the processor. This has the advantage that as well asmeasuring the oscillation amplitude, the apparatus may also control theoscillation amplitude in response to the measured oscillation amplitude.

Preferably, the output oscillation signal is input to a control devicethat controls oscillation of the object. Typically, where theoscillating object is a tip of an ultrasonic transducer, the controldevice comprises an ultrasonic wave controller.

Typically, the control device compares the oscillation amplitude with areference oscillation amplitude and controls the oscillation of theobject so that the object oscillates at substantially the referenceoscillation amplitude. Preferably, the control device controls theoscillation amplitude in real time.

Typically, the optical radiation source comprises a collimating deviceto collimate the optical radiation exiting the source.

Preferably, the width of each of the first and second optical radiationsensing areas is greater than the sum of half the width of theoscillating object and the amplitude of oscillation of the object.Typically, the amplitude of oscillation is less than the width of theoscillating object.

In one example of the invention, the first and second optical radiationsensing areas are directed towards the optical radiation source.Typically, the first and second optical radiation sensing areas areadjacent each other. The optical radiation sensing areas may becoplanar. Typically, the spacing between the first and second radiationsensing areas is not greater than 10% of the width of the oscillatingobject. Preferably, the spacing is less than 10% and is kept to minimum.

In an alternative example of the invention, the first and second opticalradiation sensing areas are not directed towards the optical radiationsource and the detector further comprises an optical device to directthe optical radiation onto the first and second sensing areas.

Preferably, the processor generates an indication of the oscillationamplitude by comparing the sum of the first and second output signalswith the difference between the first and second output signals.

Typically, the first and second optical radiation sensing areas eachcomprise a photodiode.

BRIEF DESCRIPTION OF DRAWINGS

An example of apparatus for measuring the oscillation amplitude of anoscillating object in accordance with the invention will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of an ultrasonic transducer and an opticalradiation source and detector;

FIG. 2 is a schematic view of the transducer of FIG. 1 and oscillationmeasuring apparatus incorporating the source and detector shown in FIG.1;

FIG. 3 is a block diagram of the apparatus shown in FIG. 2 with ameasurement process unit shown in more detail;

FIG. 4 is a front view of the detector shown in FIG. 1;

FIG. 5 is a cross-sectional view of the detector of FIG. 4 in use;

FIG. 6 is a cross-sectional view of another example of a detector inuse;

FIGS. 7A, 7B and 7C are schematic diagrams showing a capillary bondingtip in a central position, a left hand position and a right sideposition, respectively, with respect to the detector shown in FIG. 4;

FIG. 8 is a graph of the output signal from the detector of FIG. 4versus capillary tip position in the Y direction;

FIG. 9 is a graph of the output signal from the detector of FIG. 4versus capillary tip position in the Z direction; and

FIG. 10 shows a vibration profile of a capillary in free vibration; and

FIG. 11 shows a vibration profile of a capillary during wire bonding.

FIG. 1 shows an ultrasonic transducer 1 having a capillary 2 with a tip3. In FIG. 1 the capillary 2 is shown in a larger scale relative to thetransducer 1 for clarity and to show the shape of the capillary 2. Thecapillary 2 is located within a hole 4 in the end of the transducer 1 sothat the longitudinal axis of the capillary 2 is at approximately rightangles to the longitudinal axis of the transducer 1. The capillary 2 isremovably inserted into the hole 4 and held in the hole 4, for exampleby means of a locking screw (not shown).

The transducer 1 and capillary 2 form part of a bond head of a wirebonder for bonding conductive wire to semiconductor dies and leadframes.The wire to be bonded passes through a through bore 5 in the capillary2, which is coincident with the longitudinal axis 6, and extends out ofthe tip 3. FIG. 1 also shows a sensor head 7 comprising a body 8 onwhich is mounted an optical radiation emitter 9 and an optical radiationdetector 10.

Also shown in FIG. 1, for reference purposes only, is a set of X-Y-Zco-ordinates 11. In use, during wire bonding (i.e. during ultrasonicwelding of wire at the capillary tip 3 to a semiconductor die or aleadframe) the transducer 1 oscillates (or vibrates) in the Y directionwith respect to the sensing head 7. This vibration has been identifiedas being the most important vibration contribution to the bond quality.In addition, the transducer 1 and capillary 2 may be moved up and downin the Z direction, with respect to the sensing head 7, to bring the tip3 into contact with the surface to which the wire is to be bonded.

FIG. 2 shows oscillation measurement apparatus 12 including the sensorhead 7 (shown in phantom) with the transducer 1 and capillary 2. Theorientation of the sensing head 7 is as viewed in the Y direction shownin FIG. 1. Although in FIG. 2 the transducer 1 is shown with itslongitudinal axis extending in the X direction, this is for the purposesof clarity only and the longitudinal axis of the transducer would extendin the Y direction in use, as shown in FIG. 1. As shown in FIG. 2, theemitter 9 comprises an optical radiation source 13 and collimatingoptics 14. Hence, the emitter 9 generates a collimated beam 15 ofoptical radiation.

The detector 10 generates a first output signal V_(A) and a secondoutput signal V_(B) which are fed to a measurement process unit 18. Themeasurement process unit 18 is coupled to a system controller 19 whichin turn is coupled to an ultrasonic wave generator 20. The ultrasonicwave controller 20 generates an output 21 that is fed to the bond head22 to control the amplitude of oscillation of the transducer 1 andthereby control the oscillation amplitude of the tip 3.

FIG. 3 shows the detector 10 and the unit 18 in more detail. In thisfigure, the sensing head 7 is as viewed in the Z direction of FIG. 1.The detector 10 comprises an optical aperture 23 and two photodiodes24A, 24B located behind the optical aperture 23. FIG. 5 shows a moredetailed cross-sectional view of the detector 10 and capillary tip 3.The photodiodes 24 are mounted on a support 26. The photodiodes 24A, 24Bgenerate the first and second output signals V_(A), V_(B) respectively.The output signals V_(A), V_(B) are in the form of voltage signals whosemagnitude is indicative of the magnitude of optical radiation detectedby the respective photodiode 24.

The unit 18 includes two amplifiers 35. Each amplifier 35 receives oneof the output signals V_(A), V_(B), amplifies the signal and outputs therespective amplified signal VA, VB to a summing device 26 and asubtraction device 27. The summing device 26 sums the signals VA, VB andoutputs the sum V_(AB) (=VA+VB) to the system controller 19 and to adifference device 28. The subtraction device 27 subtracts the signalsVA, VB and outputs a signal S_(AB) (which is equal to the magnitude ofthe subtracted signals) to the difference device 28. The differencedevice 28 generates an output signal V′. This is defined as follows:$V^{\prime} = {\frac{S_{AB}}{V_{AB}} = \frac{V_{A} - V_{B}}{V_{A} + V_{B}}}$

The output signal V′ is output to a bandpass filter 29 and a lowpassfilter 30. The signals V_(A), V_(B) from the photodiodes 24A, 24Btypically comprise a DC component and an AC component. Hence, the outputV′ of the difference device 28 also includes AC and DC components.Therefore, V′ can be separated into a DC component V_(DAB) and an ACcomponent V_(AC). That is V′=V_(DAB)+V_(AC).

The lowpass filter 30 removes the AC component and so outputs the DCcomponent V_(DAB) to the system controller 19. V_(DAB) is used toposition the capillary tip 3 in the Y direction and to calibrate thesystem during assembly.

The bandpass filter 29 removes the DC component and so outputs the ACcomponent V_(AC) to an RMS device 31 which converts the AC componentV_(AC) to a DC signal V_(AAB) which is proportional to the amplitude ofthe AC component V_(AC).

In general, the output current of the photodiodes 24 is proportional tothe received power of the optical radiation. This is proportional to theeffective sensing area, assuming uniform optical radiation intensityI_(O) over the whole of the effective sensing area. The output currentis converted to a proportional voltage signal V. For the systemdescribed herein and shown in the drawings, V_(A) and V_(B) areproportional to the total sensing area of the detectors 24A, 24Brespectively. During measurement, the total effective sensing area staysconstant. Therefore, V_(AB) is proportional to the optical radiationintensity I_(O).

V_(AB) is also used as a reference signal to correctly position thecapillary tip 3 with respect to the photodiodes 24A, 24B, as describedbelow.

The system controller 19 receives the output signals from the processunit 18. Based on the parameters set in the system controller 19 and thesignals received from the process unit 18, the system controller 19calculates the necessary control parameters to drive the ultrasonic wavecontroller 20. In response to the control parameters received from thesystem controller 19, the ultrasonic wave controller 20 outputs thesignal 21 to control the amplitude of oscillation of the capillary tip 3by controlling the oscillation of the ultrasonic transducer 1.

To measure the oscillation amplitude of the capillary tip 3, thecapillary tip 3 is positioned between the emitter 9 and the detector 10,as shown in FIGS. 1 to 3. The collimated light beam 15 illuminates thecapillary tip 3 and projects a shadow of the capillary tip 3 onto thedetector 10. As shown in FIG. 4, the photodiodes 24A, 24B each have anet effective or active sensing area 25A, 25B. The sensing areas 25 facetowards the emitter 9 so that collimated light 15 entering the aperture23 is detected by the sensing areas 25. The aperture 23 is large enoughso that the shadow image of the capillary tip 3 (see FIGS. 7A, 7B and7C) is positioned within the aperture 23 during measurement but smallenough to maintain high resolution. The collimated light beam 15 must belarge enough so that part of the collimated light beam 15 passingthrough the aperture 23 projects an even illumination covering thecombined width W of the sensing areas 25 of the photodiodes 24 and theheight H of the sensing areas 25 of the photodiodes 24, as shown in FIG.4. The width W and height H, together with the separation δW of thesensing areas 25, determine the sensitivity and measuring range of theapparatus 12. The two photodiodes 24 are placed very closely behind theaperture 23. The separation δW of the sensing areas 25 is typically ofthe order of 10 μm to 100 μm. The output signals V_(A) and V_(B) areproportional to the total optical power detected by the respectivephotodiode 24A, 24B, and therefore, the proportion of the light beam 15incident on the respective sensing area 25A, 25B.

The output voltage V_(AB) generated by the summing device 26 is used asa reference signal to position the capillary in the sensor head 7.Before the capillary tip 3 is positioned in the sensor head 7, thevoltage V_(AB) is equal to U′_(SUM). A pre-defined constant β is used todetermine if the capillary tip 3 is correctly aligned in the sensor head7. When the capillary tip 3 is aligned correctly, the voltage V_(AB) isequal to U_(SUM)=βU′_(SUM) where β is a pre-defined value ranging from0.5 to 0.8 and is dependent on the expected sensitivity and measurementrange of the apparatus 12. In this way, the measurement can be made atthe same section of the capillary tip 3 for the same type ofcapillaries.

To align the tip 3 for the Y direction, firstly the voltage V_(AB) ismeasured without the tip 3 in the sensor head 7. The tip 3 is then movedinto the sensor head 7, and V_(AB) is monitored by the system controller19. When the V_(AB) starts to fall which corresponds to position Y1 inFIG. 8, the capillary tip 3 is then inserted a further distance of W/2.The position of the capillary tip can be fine adjusted by using thevoltage signal V_(DAB). When the capillary tip is positioned correctlyin the centre of the sensing areas, V_(DAB) will be equal to zero orwill be at a minimum value. The system controller 19 records this Yposition for reference as Y_(CENTRE).

To align the tip 3 in the Z direction, firstly the voltage V_(AB) ismeasured without the tip 3 in the sensor head 7. This is indicated asposition Z1 in FIG. 9. The tip 3 is then lowered into the head 7 alongthe Z direction towards the centre of the sensing areas 25 according tothe Y reference position Y_(CENTRE) while continuously monitoring thevalue of V_(AB). The capillary tip 3 is in the correct Z position whenV_(AB) is equal to U_(SUM) where U_(SUM)=βU′_(SUM). The correct Zposition is shown as position Z2 in FIG. 9, where it can be seen thatthe tip 3 partially covers the sensing areas 25A, 25B.

The Y and Z direction alignment can be done manually or automatically.Preferably, it is performed automatically by the system controller 19and bond header according to the parameters.

The oscillation of the capillary tip 3 is controlled by the systemcontroller 19 via the ultrasonic wave controller 20 which drives theoscillation of the transducer 1 in response to signals received from thesystem controller 19. When the capillary tip 3 oscillates in the Ydirection, the shadow of the tip 3 on the sensing areas 25 also moves,as shown in FIGS. 7A to 7C. In FIGS. 7A to 7C, the tip 3 has anoscillation amplitude of δY and the point of the tip 3 has an angle of2α.

Hence, the light power detected by each photodiode changes during anoscillation cycle of the tip 3 and the corresponding output signalV_(A), V_(B) changes accordingly. However, the output V_(AB) from thesumming device 26 will remain constant.

The output signals V_(A), V_(B) are fed to the processing unit 18 wherethey are amplified by the respective amplifiers 35. The signals V_(A),V_(B) are processed as described above in the processing unit 18 toobtain the three output signals V_(AB), V_(DAB), V_(AAB).

V_(DAB) indicated if the capillary tip was positioned in the centre ofwindow of sensor' receiver. V_(AAB) is directly proportional to thevibration amplitude δY of capillary tip according to following equation:

δY=_(γAC)V_(AAB)

where _(γAC) is the sensitivity of the apparatus 12. The value of _(γAC)is calculated according to the following equation:

_(γAC)=β(W−δW)/2M

where M is a constant of the processing unit 18. M is determined by theamplification of the processing unit 18. β is the pre-defined constantreferred to above and is equal to U_(SUM)/U′_(SUM). Therefore, thesensitivity of the apparatus 12 is dependent only on the width W and theseparation δW, as M and β are both constants.

An alternative example of a detector 40 is shown in FIG. 6. In thisexample the detector 40 includes two photodiodes 24A, 24B which faceeach other. The light beam 15 is reflected onto the photodiodes by areflecting device 41 mounted on a support 42. Hence, actual detectionand operation of the detector 40 is identical to that for the detector10 except that the separation δW is nearly equal to zero.

By means of the feedback system obtained by coupling the processing unit18 to the system controller 19, it is possible to adjust the oscillationamplitude of the capillary tip 3, in real time, until the amplitude isat a desired value. In addition, due to the compact nature of the sensorhead 7 it is possible to measure the oscillation amplitude of the tip 3at any time during operation of the wire bonder, including during anactual wire bond operation. This is important as the oscillation (orvibration) profile of the tip 3 is different depending on whether thetip 3 is in free vibration (i.e. not contacting a bonding surface) orperforming a wire bond operation, as shown in FIGS. 10 and 11,respectively. Therefore, this enables optimisation of the oscillationamplitude and a more consistent bonding process with a reduction in thenumber of faulty bonds.

Other advantages of the invention are that it mitigates the effect oftemperature fluctuations and does not require re-calibration before eachmeasurement. In addition, as the measurement process can be performedduring the wire bond operation it is not necessary to stop or switch offthe bonder to perform the measurement. This reduces the downtime of thewire bonder.

What is claimed is:
 1. Apparatus for detecting the oscillation amplitudeof an oscillating object, the apparatus comprising: an optical radiationsource; a detector oriented for receiving optical radiation from saidoptical radiation source, said detector comprising first and secondoptical radiation sensors adjacent each other and both receiving saidoptical radiation from said optical radiation source, the detector andthe optical radiation source being adapted to be located on oppositesides of the oscillating object from each other, with the oscillatingobject located between the source and the detector so that when solocated, the object blocks a portion of the optical radiation directedtoward said detector from the source; and a processor coupled to thedetector to receive first and second output signals generatedrespectively by the first and second sensors and representing themagnitude of optical radiation sensed by the first and second opticalradiation sensors, respectively; wherein the processor processes thefirst and second output signals to obtain an indication of the amplitudeof oscillation of the object; wherein said oscillating object is anultrasonic transducer; and wherein the width of each of the first andsecond optical radiation sensors is greater than the sum of half thewidth of the oscillating object and the amplitude of oscillation of theobject.
 2. Apparatus according to claim 1, wherein the processorgenerates an output oscillation signal that is applied to theoscillating object to modify the oscillation amplitude of the object inresponse to the oscillation amplitude indicated by the processor. 3.Apparatus according to claim 2, wherein the output oscillation signal isinput to a control device that controls oscillation of the object. 4.Apparatus according to claim 3, wherein the control device compares theoscillation amplitude with a reference value and controls theoscillation of the object so that the object oscillates at an amplitudesubstantially equal to the reference value.
 5. Apparatus according toclaim 3, wherein the oscillation amplitude is controlled in real time.6. Apparatus according to claim 1, wherein the first and second opticalradiation sensors are directed towards the optical radiation source. 7.Apparatus according to claim 1, wherein the first and second opticalradiation sensors are not directed towards the optical radiation sourceand the detector further comprises an optical device to direct theoptical radiation onto the first and second sensors.
 8. Apparatusaccording to claim 6, wherein the first and second optical radiationsensors are adjacent each other.
 9. Apparatus according to claim 1,wherein the oscillating object is a tip of an ultrasonic transducer foruse in an ultrasonic welding machine.
 10. A wire bonder comprisingapparatus according to claim
 1. 11. Apparatus according to claim 3,wherein the control device comprises an ultrasonic wave controller. 12.A method of detecting the oscillation amplitude of an oscillatingobject, said oscillating object being an ultrasonic transducer, themethod comprising the steps of: positioning an optical radiation sourceand an optical radiation detector on opposite sides of the object, thedetector being oriented for receiving optical radiation from saidoptical radiation source, the detector comprising first and secondoptical radiation sensors adjacent each other and both receiving saidoptical radiation from said optical radiation source, the oscillatingobject being located between the source and the detector so that theobject blocks a portion of the optical radiation directed toward saiddetector from said source; illuminating the object and the detector withoptical radiation from the source; and processing first and secondoutput signals generating respectively by the first and the secondoptical radiation sensors to determine the oscillation amplitude of theobject.
 13. A method according to claim 12, wherein the first and secondoutput signals are processed by comparing the sum of the first andsecond output signals with the difference between the first and secondoutput signals.
 14. A method according to claim 12, wherein theoscillating object is a tip of an ultrasonic transducer in an ultrasonicwelding machine.
 15. A method according to claim 12, further comprisingcontrolling the oscillation amplitude of the oscillating object inresponse to the determined oscillation amplitude.
 16. A method accordingto claim 15, wherein the oscillation amplitude is controlled bycomparing the determined oscillation amplitude with a reference valueand controlling the oscillation of the object to oscillate at anamplitude substantially equal to the reference value.
 17. A methodaccording to claim 15, wherein the oscillation amplitude is controlledin real time.
 18. Apparatus according to claim 1, wherein said opticalradiation sensors receive said optical radiation directly from saidoptical radiation source.
 19. Apparatus according to claim 15, whereinsaid detector is oriented facing said optical radiation source. 20.Apparatus according to claim 1, wherein said detector is oriented facingsaid optical radiation source.
 21. A method according to claim 12,wherein said optical radiation sensors receive said optical radiationdirectly from said optical radiation source.
 22. A method according toclaim 21, wherein said detector is oriented facing said opticalradiation source.
 23. A method according to claim 12, wherein saiddetector is oriented facing said optical radiation source.
 24. Apparatusaccording to claim 1, wherein the oscillating object is a tip of anultrasonic transducer for use in a wire bonder.
 25. An ultrasonicwelding machine comprising apparatus according to claim
 1. 26. A methodaccording to claim 12, wherein the oscillating object is a tip of anultrasonic transducer in a wire bonder.
 27. A method according to 12,wherein the width of each of the first and second optical radiationsensors is greater than the sum of half the width of the oscillatingobject and the amplitude of oscillation of the object.